This invention relates generally to electrical measurement apparatus, and more particularly to apparatus for measuring coating thicknesses of electrically-conductive materials by utilizing an induced eddy current method.
Eddy current methods are commonly used as a means of measuring coating thicknesses of certain materials. An example of such materials are Zircaloy samples that have been exposed to nuclear radiation, on which the thickness of a zirconium oxide corrosion film coating is to be measured utilizing the eddy current technique. The zirconium oxide coating thicknesses to be measured are usually less than 5.times.10.sup.-4 inch, which requires extremely high accuracy in thickness measurement. However, temperature changes of the Zircaloy sample caused by gamma heating have a deleterious effect on accuracy. This is due to the fact that heat causes a change in the resistance of the probe coils used in obtaining the measurement which in turn produces an imbalance in an electrical bridge circuit employed to detect the eddy current value. Such temperature-caused resistance changes can result in measurement errors greater than 100%.